An example of an environment in which the present invention is intended to operate is an arrangement for continuously casting molten metal directly into strip, e.g., steel strip. Such an apparatus typically comprises a pair of horizontally spaced rollers mounted for rotation in opposite rotational senses about respective horizontal axes. The two rollers define a horizontally disposed, vertically extending gap therebetween for receiving the molten metal. The gap defined by the rollers tapers in a downward direction. The rollers are cooled, and in turn cool the molten metal as the molten metal descends through the gap.
The gap has horizontally spaced, open opposite ends adjacent the ends of the two rollers. The molten metal is unconfined by the rollers at the open ends of the gap. To prevent molten metal from escaping outwardly through the open ends of the gap, mechanical dams or seals have been employed.
Mechanical dams have drawbacks because the dam is in physical contact with both the rotating rollers and the molten metal. As a result, the dam is subject to wear, leaking and breakage and can cause freezing and large thermal gradients in the molten metal. Moreover, contact between the mechanical dam and the solidifying metal can cause irregularities along the edges of metal strip cast in this manner, thereby offsetting the advantages of continuous casting over the conventional method of rolling metal strip from a thicker, solid entity.
The advantages obtained from the continuous casting of metal strip, and the disadvantages arising from the use of mechanical dams or seals are described in more detail in Praeg U.S. Pat. No. 4,936,374 in Lari, et al. U.S. Pat. No. 4,974,661, in Gerber, et al. U.S. Pat. No. 5,197,534, and in Praeg U.S. Pat. No. 5,251,685, each of which is hereby incorporated by reference.
To overcome the disadvantages inherent in the employment of mechanical dams or seals, efforts have been made to contain the molten metal at the open end of the gap between the rollers by employing an electromagnet having a core encircled by a conductive coil, through which an alternating electric current flows, and having a pair of magnet poles located adjacent the open end of the gap. The magnet is energized by the flow of alternating current through the coil, and the magnet generates an alternating magnetic field, extending across the open end of the gap, between the poles of the magnet. The magnetic field can be either horizontally disposed or vertically disposed, depending upon the disposition of the poles of the magnet. Examples of magnets which produce a horizontal field are described in the aforementioned praeg U.S. Pat. Nos. 4,936,374 and 5,251,685, and Gerber, et al. U.S. Pat. No. 5,197,534; and examples of magnets which produce a vertical magnetic field are described in the aforementioned Lari, et al. U.S. Pat. No. 4,974,661. The alternating magnetic field induces eddy currents in the molten metal adjacent the open end of the gap, creating a repulsive force which urges the molten metal away from the open end of the gap.
The static pressure force urging the molten metal outwardly through the open end of the gap between the rollers increases with increased depth of the molten metal, and the magnetic pressure exerted by the magnetic field must be sufficient to counter the maximum outward pressure exerted by the molten metal. A more detailed discussion of the considerations described in the preceding sentence and of the various parameters involved in those considerations are contained in the aforementioned two Praeg, Gerber, et al. and Lari, et al. U.S. Patents. As disclosed in the Praeg and Lari, et al. patents, non-magnetic, electrically conductive heat shields can be disposed between the molten metal sidewall and the magnetic poles at the open side of the gap to protect the electromagnet coil from excessive heat and to shape the magnetic flux density.
The maximum magnetic pressure P.sub.max exerted on the molten metal sidewall at the open end of the gap between the rollers by the electromagnet should be at least equal to the full static pressure head of the molten metal (melt) contained between the rollers: EQU P.sub.max =.rho.gh (1)
where
.rho. is the liquid metal density; PA1 g is the acceleration due to gravity; and PA1 h is the depth of the melt pool from the upper melt level to the end of the solidification point, at the nip.
The magnetic pressure P is related to the electromagnetic force, f/ , which is the product of the induced current j/ and magnetic induction or flux density B/ : EQU f=j.times.B (2)
In one embodiment employing horizontally disposed electromagnetic fields, the prior art achieves magnetic confinement of the sidewall of molten metal at the open end of the gap by providing a low reluctance flux path near the end of each roller (the rim portion of the rollers). The apparatus of the prior art comprises an electromagnet for generating an alternating magnetic field that is applied, via the low reluctance rim portions of the rollers, to the sidewall of the molten metal contained by the rollers. For efficient application of the magnetic field, each magnet pole must extend axially, relative to the rollers, very close to the end of a respective roller to be next to the low reluctance rim portion of the roller and separated from this rim portion by only a small radial air gap. For efficient operation, the low reluctance flux path in the rim portion of a roller usually is formed from highly permeable magnetic material.
Another expedient for horizontal containment of molten metal at the open end of a gap between a pair of members, e.g., rollers, is to locate, adjacent the open end of the gap, a coil through which an alternating current flows. This causes the coil to generate a magnetic field which induces eddy currents in the molten metal adjacent the open end of the gap resulting in a repulsive force similar to that described above in connection with the magnetic field generated by an electromagnet. Embodiments of this type of expedient are described in Olsson U.S. Pat. No. 4,020,890, and Gerber U.S. Pat. No. 5,197,534, hereby incorporated by reference.